This invention relates to a polyester film with improved dimensional stability, abrasion resistance, drawability and slip properties.
Biaxially or multiaxially oriented polyester films, for example biaxially oriented polyethylene terephthalate film, have good tensile strength, tear resistance, modulus of elasticity transparency, and chemical and thermal resistance. Due to these properties, polyester film has found wide use in a variety of applications, including video, audio, and computer film substrates, as well as in packaging and reprographic products.
Polyester films must meet specific requirements for various applications. One such requirement is a sufficiently low coefficient of friction on the film surface. The rougher the film surface is, the better is the winding behavior and therefore the production yield. In addition to a low coefficient of friction and good mechanical properties, polyester film for magnetic tapes must additionally possess a uniform surface roughness, good abrasion resistance, and dimensional stability.
These requirements can be met by adjusting the polymer recipe or the process parameters during film manufacture. Adjusting the surface topography to satisfy one of these requirements for magnetic tape base film can be achieved by either of two methods.
The first method comprises the addition of inorganic particles to the polyester. The particles may be selected from the group comprising calcium carbonate, silicon dioxide, kaolin, barium sulfate, and titanium dioxide
The second method involves creation of "internal particles" by a controlled precipitation of catalyst residues and monomers or oligomers of the polyester during the polyester synthesis.
The use of inorganic particles is likely to lead to the formation of voids during the draw process, at the places where the matrix tears away from the particle. In the further processing of the film, for example during coating, parts of the polymer covering in the area near the surface can loosen and abrade. The non-uniformity of the polyester film surface caused by such abrasion may render the film unsuitable for magnetic tape applications.
In addition, the more or less wide particle size distribution of inorganic particles, as well as the tendency for the formation of agglomerates despite painstaking preparation processes, also have a negative effect upon film quality.
The creation of so-called "internal particles" (catalyst precipitates) of defined size and quantity is extremely difficult from a process-technological point of view.
To improve the adhesion between the polymers and the particles which are added to improve the surface topography of the film, U.S. Pat. No. 4,320,207 describes the inclusion of micronized, crosslinked organic, hardened organic, globular (see DE-OS 33 13 923) spherical particles, with functional groups and narrow particle size distribution, in the polyester, instead of the inorganic particles. Films which contain the crosslinked spherical particles with narrow particle size distribution as well as functional groups for better bonding in the matrix have a defined surface roughness and therefore have good electro-magnetic properties. Since, furthermore, they should have a good slip and excellent transparency, they should be well suited as base films for packaging materials as well as technical applications such as photographic film, magnetic tape, etc., especially since their mechanical properties can be adjusted through the process parameters of the manufacturing process.
However, in practice it has been shown that the films have insufficient abrasion behavior as well as unsatisfactory dimensional stability, and therefore are unsatisfactory for many end uses.
It has also been suggested already (German patent P 34 34 838) that the dimensional stability and abrasion resistance of polyester films be improved by the addition of nucleating agents. Films of these types, however, do not have optimum slip properties.
The object of this invention, therefore, is to make a polyester film with a very uniform surface, which has good slip properties as well as excellent abrasion resistance, improved dimensional stability and drawability.